This invention relates generally to fiber optic lighting, and in particular to a method for manufacturing a fiber optic cable for use in side lighting applications.
Fiber optics is the technique of transmitting light through long, thin, flexible fibers or filaments of glass, plastic or other transparent materials. A bundle of fibers is referred to as a fiber optic cable. Conventional fiber optic cables include a core of optical fibers around which one or more layers of optical fibers are spirally wound and are enclosed within a transparent sheath. An optical fiber is a cylindrical dielectric wave guide of small diameter made of a low-loss material, usually fused silica glass or a polymer of high chemical purity and high refractive index.
Optical fiber cables are used for transmitting light for illumination purposes, either in a longitudinal mode in which light is transmitted from an originating point to a receiving point, or in a "side light" lateral operating mode in which light is emitted laterally or radially along the entire length of the fiber optic cable. Typically, fiber optic cables are operated in the lateral or side lighting mode for decorative lighting, recessed indoor lighting, around swimming pools and walkways and in indoor applications where soft accent lighting is desired.
Fiber optic cables configured for side lighting applications are constructed in lengths which may vary from a few feet to one hundred feet or more. In a typical side lighting application in which a long fiber optic cable is used, the cable is coupled at one or both of its ends to receive light from a high intensity light source. When illuminated, the optical cable glows, thus providing essentially the same side illumination pattern that would be expected to be produced by a tubular incandescent or fluorescent luminaire.
Although the side light fiber optic cable is not as efficient as a conventional fluorescent or incandescent luminaire, it offers several advantages in that the fiber optic cable can be extended remotely from the light source itself, and can be curved to conform to a particular pattern or to form a continuous border around an object or area to be illuminated. Accordingly, a single continuous length of fiber optic cable can replace multiple incandescent light fixtures, for example in area lighting around a swimming pool, patio, garden area or around a walkway. Moreover, the fiber optic cable produces a soft glow, which cannot be duplicated by incandescent or fluorescent lighting, and therefore is useful for creating special indoor lighting effects, for example hi-lighting three-dimensional art objects and wall paintings.
Conventional fiber optic cable arrangements useful for side lighting applications are relatively inefficient as compared to conventional fluorescent and incandescent luminaires. A further limitation on the use of conventional fiber optic cable is that the spiral winding pattern is visible and thus detracts from the soft, uniform glow lighting effect that is desired for many direct illumination applications in which the fiber optic cable is visible. Moreover, the spiral winding pattern is accentuated when the fiber optic cable is illuminated by a colored light source.
Another limitation on the use of conventional fiber optic cables relates to termination of the cable. The fiber optic strands must be cut and terminated cleanly without smearing or beveling the ends that are closely coupled to the light source. Conventional fiber optic cables that are spiral-wound cannot be cut cleanly, in part because the fiber strands are not uniformly centered and thus cannot provide uniform inter-strand support as the strands are being sheared.
Conventional optical fiber cables adapted for side light lateral operation are exemplified by U.S. Pat. No. 5,416,875 entitled "Optical Fiber Lighting Apparatus and Method," which discloses an optical fiber cable having a bunch-twisted, multiple strand core with multiple stranded bundles laid in a spiral pattern around the core. U.S. Pat. No. 5,617,497 entitled "Lateral Illumination Fiber Optic Cable Device and Method of Manufacture," discloses a similar winding arrangement in which multiple stranded bundles are laid in a spiral pattern around a tubular core.